For tone-based channel monitoring in a wavelength division multiplexing (WDM) communications system, each wavelength is modulated with one or more tones that are specific to the wavelength. U.S. Pat. No. 7,054,556 to Wan et al. describes a scheme in which channels in an optical WDM system are each modulated by two or more alternating dither tones so that at any instant, each channel is modulated by at least one dither tone. As described therein, channel monitoring by detecting the dither tones makes use of a Fast Fourier Transform (FFT) process which can decode and measure dither tones encoded on the WDM channels.
In carrying out such non-intrusive real-time channel monitoring, a tone decode subsystem (also referred to as a wavelength tracker) is used. In a wavelength tracker, the optical WDM signal is typically converted to an electrical signal using a photodiode. The tones in the converted electrical signal and their respective levels provide information regarding which wavelength channels are present in the WDM signal as well as the optical power levels of the channels present. This information is critical to the WDM system for channel monitoring and power management. For example, wavelength channels whose power levels are monitored to be below (above) the desired level can be boosted (attenuated), e.g., via gain equalization in an optical amplification stage, in order to achieve good transmission performance of all the channels.
To monitor the powers of the wavelength channels in a WDM system, the operation of a conventional wavelength tracker detector is based on the simultaneous measurement of the powers of the different frequency tones that are assigned to different WDM channels. Stimulated Raman scattering (SRS) during optical fiber transmission, however, has the effect of transferring energy from shorter wavelength channels to longer wavelength channels, especially when the powers of these channels are high and/or the transmission distance is long. Such energy transfer is also called SRS-induced channel crosstalk which causes an appreciable portion of the frequency tone that is originally assigned to a given wavelength channel to be transferred to other WDM channels. This makes a conventional wavelength tracker inaccurate in reporting the power level of each channel and subsequently causes power tilt and degradation in optical signal-to-noise ratio (OSNR), thereby negatively affecting system performance. In some cases, the inaccuracy may become so large that the wavelength tracker will incorrectly report the presence or absence of a wavelength channel.
It is thus desired to improve the tolerance of tone-based wavelength tracking to SRS so that accurate optical channel monitoring can be achieved even with high signal power and system reach.
There is no known solution to the above-described problem. A possible approach towards remedying this problem is to estimate the tone transfers among the WDM channels in each fiber span, and calibrate the measured power for each frequency tone to reflect the actual power of the wavelength channel to which the tone frequency is assigned. This approach, however, requires the knowledge of the channels transmitted in each fiber span such as the locations and input powers of the channels, the tone components currently carried in each channel, and the fiber nonlinear and loss coefficients. The tone transfers in the fiber span then need to be computed, which is computationally intensive.
Moreover, the powers of the frequency tones impressed on a given channel due to the SRS need to be recorded and this information passed with the channel for further computation at the next wavelength tracker. This becomes impractical to realize in transparent WDM systems using reconfigurable optical add/drop multiplexers (ROADMs), where a wavelength channel can be added, dropped, or re-routed on demand.